In a typical conventional integrated circuit voltage regulator such as that shown in FIG. 1, an error amplifier (amplifier 1) is employed to compare a reference voltage VR (which is a 1.25 volt signal in the FIG. 1 example) with a signal proportionaly to the output voltage. The amplifier output controls a transistor (PNP transistor 2) through which the output current flows, by adjusting the transistor so that the output voltage (Vout) equals a fixed multiple of the reference voltage (kVR). A source voltage (sometimes referred to herein as an "input voltage"), which may be unregulated, is supplied to the emitter of the transistor.
A voltage regulator may be characterized by its "dropout voltage," which is the lowest source voltage which will allow the regulator output voltage to remain substantially constant at kVR, the above-mentioned fixed multiple of the reference voltage. If the source voltage drops below the dropout voltage, then the regulator's output voltage will decrease below kVR.
In the conventional FIG. 1 circuit, a redundant source voltage (V IN) is supplied to the emitter of transistor 2. The source voltage is redundant in the sense that the source voltage is normally supplied through diode D from a battery (and hence is denoted V BAT), but if the battery fails, the source voltage supplied to transistor 2 is the time-varying voltage V.sub.c across capacitor C.sub.1 (V.sub.c varies as capacitor C.sub.1 discharges). The use of diode D for switching to the redundant input voltage V.sub.c is undesirable because it causes the FIG. 1 circuit to have a high dropout voltage.
It has not been known until the present invention how to construct a low dropout voltage regulator with a switched redundant input, which will have low dropout voltage before, during, and after a switch to the redundant input.